1. Field of the Invention
The present invention relates to a method for fabricating an optical disc master, an apparatus best adapted to carry out the optical-disc-master fabricating method and an optical disc produced by a master produced by the optical-disc-master fabricating method.
2. Description of the Prior Art
In the case of optical discs of a type capable of recording additional data by users and reading out data, e.g. add-on writable optical discs, each optical disc is in general formed with tracks for permitting tracking of a read/write head or prepits for giving information signals.
As to types of tracks, there exist grooves used in a continuous servo system and wobble marks used in a sampled servo system.
Therefore, the term "tracks" is used to include grooves and wobble marks in this specification and the term "track pattern" is used to refer to a pattern recorded by a plurality of tracks or prepits.
In general, track patterns are divided into spiral pattern and concentric pattern.
One continuous spiral can be recorded by exposing a rotating substrate with a uniform displacement either of an optical system including a write head or the rotating substrate.
On the other hand, in the case of the concentric pattern, the tracks are discontinuous so that the relative displacement between the write head and the substrate cannot be carried out continuously simultaneously with the exposure.
In the case of a conventional method for forming a concentric track pattern, the relative displacement between the write head and the substrate and the exposure are carried out intermittently so that the exposure during one rotation of the substrate and the displacement by one pitch of a track on the substrate are alternately carried out.
More particularly, in the case of the conventional method for forming the concentric track pattern of the type described above, the relative displacement between the write head and the substrate is carried out by an intermittent energization of a motor so that there arise problems in that correct positioning is difficult and the degree of pitch accuracy is lower as compared with formation of a spiral track pattern.
As to methods for recording prepits, there are two methods and according to one method, prepits are recorded in a groove while according to the other method prepits are recorded on a land between adjacent grooves. Of various conventional methods for fabricating an optical disc master in which prepits are recorded on the land between adjacent grooves, there has been proposed and demonstrated a method in which recording is made by two light beams for grooves and prepits, respectively.
FIG. 14 shows a schematic construction of an apparatus adapted to carry out the conventional method for fabricating a master disc. According to this conventional method for producing a master disc, a substrate 15 is rotated at a predetermined rotational speed and displacement in a radial direction of the substrate 15 is carried out continuously at a predetermined speed at which the displacement is made by a predetermined pitch during one rotation of the substrate 15. A light beam emitted from an argon laser 11 is split by a half mirror 22 into two beams for recording grooves and prepits, respectively. The split beams are modulated in response to predetermined signals, respectively, in AO (Acousto-Optic) modulators 12 and then combined by a polarized beam splitter 26 so as to be applied to a write head 14 having a focus servo mechanism employing a semiconductor laser. The optic axes of the two light beams are so adjusted that they are focused on the substrate at the positions spaced apart from each other by one half of a predetermined pitch.
In the case of the master fabricating method of the type described above, the reflecting directions of the mirror must be adjusted so that the light beam for grooves and the light beam for prepits must be on the substrate at two points spaced apart from each other by, for instance, 0.8 .mu.m which is one half of a predetermined pitch with a positional accuracy of .+-.0.03 .mu.m. Therefore, the apparatus becomes complicated in construction because one laser beam must be split into two beams and a high power recording laser is necessary so that the apparatus becomes very expensive.
Meanwhile, Japanese Patent Application Laid-Open No. 62-84449 discloses a method for recording a plurality (above three) of grooves simultaneously.
FIG. 15 shows a fundamental construction of a conventional apparatus capable of recording a plurality of grooves simultaneously. A laser beam emitted from a laser source (not shown) is made to pass through a expander (not shown) and a lens system (not shown), thereby obtaining a plurality of parallel beams 51. Thereafter, as shown in FIG. 16, these parallel beams 51 are made to pass downwardly through a portion which establishes multiple interference conditions by two reflecting plates 52 and 53 which intersect each other at an angle of inclination .phi.. In this case, the angle of inclination between the two reflecting plates 52 and 53 is suitably selected in response to the wavelength .lambda. of the laser beam to be used so that an interference waveform having a period corresponding to a predetermined space Q between the adjacent grooves can be obtained. The interference waveform is shaped by a slit 54 and, furthermore, is mono-axially focused through a cylindrical lens 55. As a result, a plurality of bright points of light 57 are focused in the form of a linear array on a focal plane 56. It follows therefore that when the bright points 57 are focused on the substrate 15 which is rotated, a plurality of grooves can be simultaneously recorded.
According to the above-mentioned conventional method for recording simultaneously a plurality of grooves, the laser beam is split into a plurality of bright points and the energy of the laser beam is lost by absorption of light in the optical system shown in FIG. 15 so that a high-power laser is needed. Furthermore, each of the bright points cannot be modulated independently, and there exists a defect that it is difficult to record prepits.